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Single- vs. dual-supply op amps

Discussion in 'Electronic Design' started by Stephen Boulet, May 24, 2005.

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  1. I was wondering, what are the downsides to using a dual-supply op amp in
    a single-supply configuration?

    Stephen
     
  2. Bob Monsen

    Bob Monsen Guest

    Single supply opamps are designed so input and output can be near
    ground. 'normal' opamps often can't get near the power rails, so using
    them near ground doesn't work out so well.

    However, if you are using a rail-to-rail opamp, it should be ok.
     
  3. Really there's no such thing as a single supply op-amp. If they run
    off a single supply, they are biased to the half way point, so
    in effect they do see a dual supply. Run it off a 12v supply,
    and bias it to 6volts, and the opamp sees 6volts on the positive
    supply line and -6volts on the negative supply line, relative to
    that bias point. This isn't any different from running the opamp
    off a -6/+6 power supply, since the opamp will have 12 volts across
    it.

    One disadvantage of running an op-amp off a single supply is
    that the input and output will not be at ground, but at half of
    the supply voltage (as a result of the biasing). So there won't
    be zero DC volts on those lines (unless you measure in reference
    to the bias point), and so DC coupling may be out of the question
    depending on applications.

    The real thing is that "single supply op-amps" are designed so
    the output can go closer to the supply voltage. This is useful
    since if you're running it off a single supply, usually that is
    a lower voltage than the sum when you've got a bipolar power supply,
    12volts versus 24volts. The greater headroom of the "single supply
    op-amps" means they work better at those lower voltages.

    Michael
     
  4. Guest

    To put it more technically, the input common mode range of a single
    supply op amp pretty much has to include the negative rail, and the
    output stage has to be able to sink a milliamp or so when the load is
    only of the order of a hundred millivolts more positive than the
    negative rail.

    If input and output don't have to get within a few volts of the
    positive or negative rails, there isn't any technical necessity to use
    a single supply op amp. Since the LM324 is probably the cheapest op amp
    around, you will find it used in situations that don't really call for
    a single supply amp, but that's economics ....
     
  5. Guest

    To put it more technically, the input common mode range of a single
    supply op amp pretty much has to include the negative rail, and the
    output stage has to be able to sink a milliamp or so when the load is
    only of the order of a hundred millivolts more positive than the
    negative rail.

    If input and output don't have to get within a few volts of the
    positive or negative rails, there isn't any technical necessity to use
    a single supply op amp. Since the LM324 is probably the cheapest op amp
    around, you will find it used in situations that don't really call for
    a single supply amp, but that's economics ....
     
  6. James Meyer

    James Meyer Guest

    That last statement is wrong. Single-supply opamps do not need to be
    biased if they have input and output voltages that are not outside the rail
    voltages. They can be biased, but it is not because that's the only way to make
    them work.

    Jim
     
  7. Pooh Bear

    Pooh Bear Guest

    Depends what you're doing with it. There may be no downside at all.

    Graham
     
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